Large scale permanent magnet with central cavities are useful for magnetic resonance imaging and other applications. The costs of manufacturing such a magnet for a given field strength, particularly at the high end, are extreme. The use of superconductivity to produce desired field strengths is also less desirable than permanent magnets due to the more complex physical requirements involved in such structures. In designing a permanent magnet for use in magnetic resonance imaging, dimensioning a shape for practical use in a surgical suite or hospital environment should be of prime concern.
A basic requirement of the design of a permanent magnet is the efficient use of the energy stored within the magnetized material. In general, the design should be aimed at achieving the desired value of the intensity within the cavity of the magnet with a minimum volume and weight of the magnetized material.
Permanent magnets can be classified in two categories: yokeless magnets where the magnetic structure is designed in such a way that the field is confined within the magnet without the need of a high magnetic permeability yoke, and yoked magnets where the magnetic material is used to generate the desired field within the region of interest and the field confinement is achieved with an external yoke.
Yokeless permanent magnets can be designed with magnetic materials which have a quasi-linear demagnetization characteristic with a slope close to that of air. In this case the magnetic structures are transparent to the field generated by other sources. This is an important property of the yokeless structures, which allows the designer to increase the strength of the field within the cavity by superimposing the fields generated by concentric magnets.
Yoked permanent magnets, on the other hand, are shielded from external sources by the same yoke which confines the field of the magnet. As a consequence, the field superposition property of yokeless structure does not apply to yoked magnet and the field strength attainable within the cavity has an upper limit dictated by the magnet geometry. Conversely, because the magnetic material performs only the function of generating the field within the cavity, in general yoked structures use less magnetic material than yokeless structures designed for the same field strength and the same geometry of the cavity. Thus weight of the magnetic material may become an important factor in the choice between a yokeless and a yoked magnet design, particularly if the design parameters dictate the use of high energy, high cost magnetic materials.
The following publications, which are all publicly available at the Library of New York University School of Medicine, are hereby incorporated by reference and form a part of this application:
1. M. G. Abele. Linear Theory of Yokeless Permanent Magnets. EMMA '89, Rimini, Italy, 1989. PA0 2. M. G. Abele. Design of Yokeless Rare Earth Magnets for NMR Medical Applications. Proceedings of 10th International Workshop on Rare Earth Magnets, Kyoto, 1989, pp. 121-130. PA0 3. M. G. Abele. Some Considerations about Permanent Magnet Design for NMR. TR-13, New York University, N.Y., N.Y. Feb. 1, 1986. PA0 4. M. G. Abele. Design of Two-Dimensional Magnets without Magnetic Yoke. TR-15, New York University, N.Y., N.Y. Mar. 1, 1987. PA0 5. M. C. Abele. Yokeless Permanent Magnets. TR-14, New York University, N.Y., N.Y. Nov. 1, 1986. PA0 6. M. G. Abele. Three-Dimensional Design of a Permanent Magnet. TR-16, New York University, N.Y., N.Y. Jun. 1, 1987. PA0 7. M. G. Abele. Use of Materials of Different Magnetic Permeabilities in Permanent Magnets. TR-17, New York University, N.Y., N.Y. Aug. 1, 1987. PA0 8. M. G. Abele. Properties of the Magnetic Field in Yokeless Permanent Magnets. TR-18, New York University, N.Y., N.Y. Mar. 1, 1988. PA0 9. M. G. Abele. Generation of a Uniform Field in a Yokeless Permanent Magnet for NMR Clinical Applications. TR-19, New York University, N.Y., N.Y. Jul. 1, 1988. PA0 10. M. G. Abele. Generation of a Uniform Field in a Yokeless Permanent Magnet for NMR Clinical Applications. TR-19, New York University, N.Y., N.Y. Jul. 1, 1988. PA0 11. M. G. Abele. Geometric Invariance of Yokeless Two Dimensional Magnets. TR-20 , New York University, N.Y., N.Y. Mar. 1, 1989.
References in the text of this specification to previous publications followed by a numeral from 1 to 5 are intended to mean references 1 to 5 listed above. Previous publications (1, 2) have presented the design methodology for two-dimensional as well as three-dimensional yoked and yokeless magnetic structures capable of generating a uniform field within a cavity of arbitrary geometry. This application focuses on the problem of design optimization by discussing the properties of a figure of merit which characterizes the geometries of both categories. The analysis is confined to two-dimensional geometries and introduces the concept of hybrid magnet configurations whose design is optimized by taking advantage of the properties of both yoked and yokeless structures.